Some microelectromechanical (MEMS) accelerometers measure acceleration by sensing the inertial forces applied by a proof mass on flexible mechanical anchors. One technique of reading out the force, and therefore the input acceleration, is to measure the displacement of the mass relative to a frame. Another technique is to measure the stress induced in the restoring anchors as they counteract the inertial forces. In some MEMS accelerometers, these measurements are sometimes made by sensing changes in capacitance which communicate the changes in displacement or stress. These changes in capacitance may also be sensed through changes to optical fields. The MEMS accelerometer is typically calibrated once, which occurs in the factory and before use. In the calibration, the output signal of the accelerometer device is measured when the device is subjected to a known acceleration, and the output is corrected to a standard output value.